Efficient Server Design for Hybrid Multicast Streaming - Scalable Continuous Media Streaming Systems - page 358

T R T R T R T R

static channel 1
static channel 2
static channel 3

Mi Micro-round round

Time

All channels in the same group
start transmit at the same time

A macro round A macro round

Transmission

Static round Dynamic round Static round

… …

Data Retrieval

Data for all the
requests in a group

Micro-round

Figure 20.2 Transmission and retrieval schedule of GSS

macro-round). Reducing G we can pack more video streams in a group and this results in
increased disk efficiency, albeit at the expense of increased buffer requirement and scheduling
delay and vice versa. In the extreme case with G = 1, GSS reduces to SCAN; and in the other

extreme case with G n , where n is the maximum number of streams that the system can

support, GSS reduces to first-come-first-serve.
In SS-VoD, static channel starts once every T R seconds, where T R is given by

=

L
N S

T R = (20.1)

and L is the length of the video title in seconds. However, transmission in GSS can only
start at the beginning of a micro-round and hence does not necessary match the transmission
schedule of the static channels. In particular, when T R is not divisible by the micro-round
time, some of the static channels will not be able to transmit precisely at the scheduled time.
To avoid this problem, we can use additional buffers to perform read-ahead to absorb the
time differences. However, this will increase the buffer requirement up to 50% and is thus not
desirable. Alternatively, we can choose the value of G and Q so that T R is an integer multiple
of the duration of a micro-round to avoid the additional buffer requirement. For each video
stream, N data blocks, one from each disk, are retrieved in a micro-round for transmission
over the next G micro-rounds. Thus, the service round length T r is given by

NQ
R V ,

T r = (20.2)